Oil and gas pipelines are usually formed from many lengths of externally coated steel pipe which are welded together before they are laid. The pipes are coated to prevent corrosion and are usually coated at a factory remote from the site where they are to be laid. This is generally more cost effective than coating them at the site. At the factory coating is applied to the outside of the pipes leaving a short length, about 15 cm, uncoated at each end. This is necessary to enable the pipes to be welded together and to end on site to form a pipeline. Each resulting girth weld and adjacent uncoated region of the pipe must be coated before the pipeline is laid.
Many types of coating techniques are known. The present invention is concerned with coatings which are sprayed onto the pipeline as a liquid and subsequently set, for example a two part polyurethane coating. This is supplied as two liquids which when mixed together set chemically. Conventionally liquid coatings are applied to girth welds manually using conventional spray guns. This is time consuming. Pipeline joints need to be coated at the rate the pipelin is being laid. Commercial pressure necessitates that this happens as quickly as possible. In some instances up to 500 joints must be coated per day. Several operatives are required to achieve this, which is costly. It is difficult for operatives to apply an even coating all around the surface of a pipe, especially when under considerable time pressure. Often the top and sides of pipes received a much thicker layer of coating than the underside. This is undesirable. The coatings used are often toxic and present an environmental hazard. It is preferred that operatives are not exposed to the coating and that as little as possible enters the environment. With manual spraying it is found that a significant fraction of the coating used enters the environment as overspray and through flushing the spraying apparatus.
It is an object of embodiments of the present invention to address these and other problems associated with manual coating of pipeline welds on site.
Apparatus for coating pipes is know. U.S. Pat. No. 5,207,833 discloses a machine for applying a protective coating to a pipe or pipeline as the machine travels down the pipe. The machine comprises a two piece yoke which separates at the top and bottom to enable it to be fitted around the pipe. Each of the two pieces of the yoke serve as a track on which a spray gun moves, one spray gun flexibly mounted on each of the two pieces. The spray guns are aimed at the pipe and are moved up and down opposite each other by means of a gear b it drive mechanism. As this machine is for line coating of pipe it is generally unsuitable for coating girth welds on site and also has a number of other drawbacks. In particular it is rather cumbersome and would therefore be difficult to rapidly mount and dismount on a pipe, as required when coating welds on site. The materials used to coat field welds often cause blockages within spray guns. With two spray guns the risk of a blockage occurring is double that of a single gun. The two gun arrangement also necessitates either complex valving so that they operate individually, or a second pumping system. This is inconvenient and/or expensive. Also, because each gun can only travel through 180° to apply a coating all around the outside of a pipe it is necessary for them to tilt at the end of each movement. This is also inconvenient and increases losses due to overspray. Having two guns also means that there are two points at which the coating applied to a pipe will overlap increasing the risk that the coating will run or sag. If the output of the two guns is not carefully matched coating will not be evenly applied all over the pipe. Two guns also increases wastage of coating when the spraying equipment is flushed. This is a particularly important consideration when coating welds on site as the spraying apparatus needs to be flushed between each weld. The machine includes deflectors to divert the flow of liquid away from the guns during flushing upon which the flushing liquid will enter the environment. This is undesirable. Indeed, in some countries contemporary environmental concerns are such that such operations would be forbidden. Fumes may also enter the environment during both spraying and flushing.
It is further object of embodiments of the present invention to provide an apparatus and method which addresses the problems associated with the machine described in U.S. Pat. No. 5,207,833. It is a still further object to provide apparatus for coating pipeline welds on site more quickly, conveniently end with less environmental impact and coating waste than hitherto.
GB 2285592 discloses apparatus for spray coating a non-rotating pipe comprising a support frame carrying a rotary frame, and a spray head mounted on the rotary frame. The rotary frame is free to rotate through 360°.